1. Field of the Invention
The present invention relates generally to in-mold coatings and more particularly to the use of a radiation curable coating for the in-mold coating of peroxide-cured urethane elastomers. Still more specifically the present invention relates to an in-mold coating which includes a combination of monomers containing acryloloxy groups and at least an N-vinyl imido group.
2. Description of the Prior Art
Peroxide curable urethane elastomers are known to give properties similar to unsaturated polyester resins with the same ease of handling. Such urethane elastomers involve fully reacted urethane polymers that contain vinyl unsaturation, and the compositions are usually dissolved in vinyl monomers. Such urethane elastomers eliminate problems associated with other elastomer materials prepared from urethane resins in that they foam if moisture is encountered and the toxicity associated with free isocyanates is eliminated. Peroxide cured urethane elastomers are described in an article appearing in Rubber & Plastics News, June 29, 1987, pp. 18-19 and entitled "Peroxide-Cured Urethanes Offer Moca-Free Systems". The article was co-authored by the present inventor and two co-workers at Freeman Chemical Corporation.
While peroxide cured urethane elastomers offer substantial advantages in material handling and preparation and desirable properties for some applications, the optically clear aliphatic isocyanate-based urethane elastomers have excellent properties for the preparation of glass lenses and in particular for the preparation of lenses for eye glasses. However, the materials are subject to scratching and attack by a variety of solvents with which the materials may come into contact.
In unrelated chemical literature to that previously discussed, it is known that coatings which are radiation curable and which contain acryloloxy monomers and N-vinyl imido monomers can be applied as protective coatings for substrates, including eye glass plastics. U.S. Pat. No. 4,319,811 issued Mar. 16, 1982 to Tu, et al and entitled "Abrasion Resistance Radiation Curable Coating" describes such materials wherein the acryloloxy monomers consist of tri-acrylate and tetra-acrylate materials mixed with vinyl pyrrolidone or vinyl caprolactam. The patentee in this patent discloses the coating of plastic lenses made from cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate, polycarbonate, polystyrene, methyl methacrylate, copolymers of styrene and methyl methacrylate, and allyl diglycol carbonate. Other plastic substrates are also disclosed, but the patent does not indicate that the coating of that invention can be used for polyurethane elastomers which are peroxide curable. Furthermore, as disclosed in the Tu, et al patent, the coating process is done by conventional techniques such as dip, spin, spray, curtain, gravure and roller coating techniques.
The assignee of the present invention has also discovered the advantageous properties of materials containing five and six acryloloxy groups per molecule and at least one N-vinyl imido containing monomer. A photoinitiator is provided if the cure is to be by ultraviolet light, while no photoinitiator is required if the cure is to be by electron beam. Preferably, the composition includes 65-95% of the polyfunctional material and from 5-35% of the imido group containing monomer. Films of such materials were found to have properties superior to those of the tri- or tetra-functional materials of the prior art and have the further advantage of reducd skin and eye irritation when compared to materials having a lower functionality.
It has not previously been suggested in the work done by the assignee of the present invention, or by the Tu, et al patent, that the radiation curable coatings could be advantageously used in an in-mold coating process to increase the scratch resistant and solvent resistant properties of peroxide curable urethane elastomers. A coating which provides such benefits would represent a substantial advance in the art.